Carbon nanotubes (also abbreviated below as “CNTs”) are being studied for potential use in a wide range of fields as a key nanotechnology material. Applications are broadly divided into methods which involve the use of individual CNTs themselves as transistors, microscope probes and the like, and methods which involve the collective use of a large number of CNTs in bulk, such as in electron emission electrodes, fuel cell electrodes, and electrically conductive composites in which CNTs are dispersed.
In cases where individual CNTs are employed, use is made of, for example, a method in which CNTs are added to a solvent and the mixture is ultrasonically irradiated, following which only the CNTs that are individually dispersed are removed by a technique such as electrophoresis.
On the other hand, in an electrically conductive composite which uses CNTs in bulk, it is essential for the CNTs to be properly dispersed in the polymer or the like that serves as the matrix material.
However, CNTs generally are difficult to disperse. Because conventional composites are used with CNT dispersion therein incomplete, the attributes of the CNTs are probably not fully manifested.
Moreover, this problem also has something to do with making various applications of CNTs difficult to achieve. Hence, various methods for enhancing dispersibility which entail, for example, the surface modification of CNTs or chemically modifying the CNT surfaces, are being investigated.
One such method for dispersing CNTs involves depositing poly((m-phenylene vinylene)-co-(dioctoxy-p-phenylene vinylene)) having a coil-like structure on the CNT surface (see, for example, Patent Document 1).
Although the foregoing publication shows that it is possible to individually disperse CNTs within an organic solvent and that the polymer is deposited on single CNTs, once dispersion has been carried out to a certain degree, aggregation arises and the CNTs are captured as precipitate. Hence, it has not been possible to maintain CNTs in a dispersed state for a long time.
Methods that have been proposed for resolving the above problem include using poly(vinyl pyrrolidone) to disperse CNTs in an amide-based polar organic solvent (see, for example, Patent Document 2), and using poly(vinyl pyrrolidone) to disperse CNTs in an alcohol-type organic solvent (see, for example, Patent Document 3).
However, in the foregoing art, the polymer used as the dispersant is characterized by being a straight-chain polymer; no findings concerning highly branched polymer have been reported.
Methods that focus on highly branched polymers as the CNT dispersant have also been disclosed (see, for example, Patent Document 4). Here, “highly branched polymer” refers to a polymer having branches within the skeleton, such as star polymers, and also dendrimers and hyperbranched polymers, both of which are categorized as dendritic polymers.
These highly branched polymers exhibit distinctive shapes which, in contrast with conventional polymers of ordinary string-like shape, exhibit distinctive shapes that have a relatively sparse interior and a particle-like on the ground that branches are deliberately introduced, and also have a large number of ends that can be modified by the introduction of various functional groups. By utilizing these characteristics, there is a possibility of dispersing CNTs to a high degree compared with straight-chain polymers.
However, in the art of Patent Document 4 which uses the above-described highly branched polymer as a dispersant, in addition to mechanical treatment, thermal treatment is also required in order to maintain the individually dispersed state of the CNTs for an extended period of time. Moreover, the ability to disperse CNTs is not all that high.
Also, in the art of Patent Document 4, the yield when synthesizing the dispersant is low. Because a large amount of metal catalyst must be used as a coupling agent in order to improve the yield, there is a risk of residual metal ingredients remaining within the highly branched polymer, which may limit use in applications as composites with CNTs.
Moreover, it is known that the electrical conductivity can be improved by doping the CNTs with bromine, potassium, water, nitric acid or sulfuric acid (see, for example, Non-Patent Document 1), and that the electrical characteristics and the mechanical characteristics can be improved by doping with thionyl chloride (see, for example, Non-Patent Document 2).
However, stably increasing the electrical conductivity has been difficult because these dopants often volatize or decompose under elevated temperature or migrate within the composition.
One approach that has been proposed for resolving such problems involves using the ion-conductive resin Nafion® to disperse the CNTs (see, for example, Patent Document 5).
However, although Nafion® exhibits a relatively high conductance, it does not have such a high ability to disperse CNTs.